Method and apparatus for melting reactive materials



1970 A H. F. STERLING EIAL 3,549,353

METHOD AND APPARATUS FOR MELTING REACTIVE MATERIALS Filed Dec. 22, 19662 Sheets-Sheet 1 D 2 ii 3 5 c H g 5 g H 0 ii 0 L :1 /4 I I F/ I? 5 l2 l3M'VE/WDRS) H. F. Sterling-W.R.Georgc 40-1 BY 161 4 w h K7 10PM Y Dec.22, 1970 H. F. STERLING EIAL 3,549,353

METHOD AND APPARATUS FOR MELTING REACTIVE MATERIALS Filed Dec. 22, 1966.2 Sheets-Sheet 2 Inventor. H.F.Ster1ing-W.R.George 40-1 A Home y UnitedStates Patent O 3,549,353 METHOD AND APPARATUS FOR MELTlNG REACTIVEMATERIALS Henley Frank Sterling and Wilbert Ridd George, London,England, assignors to International Standard Electric Corporation, NewYork, N.Y., a corporation of Delaware Filed Dec. 22, 1966, Ser. No.603,841 Claims priority, applicatgog great Britain, Jan. 28, 1966,

66 Int. c1. C221: 61/04 US. Cl. 7584.1 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND OF INVENTION The term metal, whenever used in the followingspecification and in the claims includes alloys besides the individualmetals, and the term reactive is taken to mean reactive at temperaturesof the order of the melting point and above with refractory material.

Examples of materials which are reactive when molten or when raised to ahigh temperature are such metals as nickel, titanium, zirconium, iron,chromium, molybdenum, tungsten and uranium, and some semiconductors ofwhich silicon is a conspicuous example.

Because of their relatively high melting points, their resistance tocorrosion, their comparatively low densities, and their high strength,certain metals are useful for making machine parts of various kinds. Forexample, titanium, zirconium, chromium based alloys and nickel basedalloys have been found useful for forming compressor blades for turbinesand jet engines.

The melting points of metals such as nickel, titanium, zirconium,chromium, molybdenum and tungsten or metals having like or similarcharacteristics, as well as some of the carbides, oxides and otherparticular compounds of such metals, are high enough to make melting inconventional refractory crucible-type furnaces without contaminationextremely difficult and generally unsatisfactory because of the tendencyof the metals to react chemically and physically with the refractorymaterial with consequent contamination of the metal and change in itsmechanical and chemical properties. Moreover, at elevated temperatures,metals are readily reactive with the oxygen and nitrogen of the air. Thehandling of metals at temperatures from about 1300 C. to above 3000 C.,when such metals are melted in the conventional crucible type of furnacepresents problems of such dilficulty as to preclude the use of suchcrucibles at these temperatures, if reasonable purity and repeatabilityof composition is required.

For the melting of titanium and its alloys or zirconium, an arc, struckwithin a water-cooled copper vessel has been used. However, an arc canintroduce contamination from the electrodes into the metal.

SUMMARY OF INVENTION The present invention provides apparatus and amethod for melting reactive material comprising:

(a) placing the material in a container formed from two or more membershaving hollow walls and being See made of a metal having a thermalconductivity of not less than 0.49 gram calories per cm. per cm. perdeg. C. per sec. and a specific electrical resistance of not more than2.665 microhms per cm. at 0 C., said members being electricallyinsulated from one another and so positioned with respect to each otherthat a cavity is formed within said container;

(b) circulating a cooling fluid through said walls and melting the saidmaterial by means of eddy currents induced therein solely by means ofhigh-frequency electric currents flowing in the walls of the saidcontainer; and

(c) on completion of melting, allowing the said material to cool withinsaid cavity of the container such that a solid form of said material isproduced, or extracting the material therefrom.

DESCRIPTION OF DRAWINGS The foregoing and other features of theinvention will be evident from the following description of anembodiment thereof taken in conjunction with the accompanying drawingsin which:

FIG. 1 shows in cross section an apparatus suitable for melting andcasting reactive materials;

FIG. 2 shows in perspective a container formed from two identicalmembers and suitable for melting and casting reactive materials;

FIG. 3 is a sectional plan view of part of the apparatus shown in FIG.1, taken at the plane indicated by the numerals 33;

FIG. 4 shows (in sectional plan view) a formation of container membersalternative to that shown in FIG. 3; and

FIG. 5 is a diagrammatical representation of the highfrequency currentflow within said container members which form an inductance orhigh-frequency heating work coil.

DESCRIPTION OF EMBODIMENTS With reference to the drawings, the containerto hold material to be melted is formed from two hollow containermembers 10 having a molded inner portion 11 and constructed from copper,silver or gold; preferably copper with silver plate. Each containermember has an inlet tube 12 extending through most of the length of theinner cavity, and an outlet tube 13, said tubes providing means wherebya cooling fluid may be continuously circulated.

The container members are held within a further container 14, forexample a silica tube, the inlet and outlet tubes 12 and 13 extendingthrough apertures in a base plate 15 of brass and being hard soldered tothe said base plate, such that the container members are rigidlysupported within the tube 14 to be normally as near as possible to eachother without contact. The inner portions of the respective containermembers form a cavity 16 within which material may be melted and thencecast. Contact between the container members 10 may be avoided by placingbetween the members a thin sheet of insulating material 28 (FIG. 3) forexample mica. Alternatively, each member may be constructed with alongitudinal fin 17 (FIG. 4) having apertures through which screws 29 ofinsulating material, for example, nylon may pass. The screws extend intocorresponding apertures in the body of the other container member,thereby iniulating all parts of one container member from the ot er.

The preferred shape of the container members 10 is shown in FIG. 3.Light and heat from the melt within the cavity 16 are prevented fromreaching the insulating material, which would otherwise be susceptibleto decomposition. With reference to the construction of the coni n untainer members as shown in FIG. 4, heat and light from the melt willimpinge on the walls of the tube 14. If the latter were made of glassand not quartz, the heat developed would produce a crack in the glass.Therefore, it is desirable to construct the container members in theform shown in FIG. 3 if there is danger that the outer container willdeform or crack when subjected to heat from the melt.

The tube 14 is held between the base plate 15 and an upper plate 18, forexample brass, by means of two or more fiber glass rods 19, each passingthrough apertures in each plate and having screw threads at each end. Bytightening the nuts on the screw threads, the tube 14 is held firmly inposition. Gas tight seats 21 are provided at the interface between theedges of the tube 14 and the plates such that a particular gas pressureor degree of vacuum may be constantly maintained within the regiondefined by the tube 14 and plates 15 and 18.

Radio-frequency or high-frequency current is supplied to the insidesurface of the container by means of inductive coupling from a primarycoil in the form of a hollow helical copper coil 22 heat insulated fromand encircling the tube 14. Coil 22 may cover an area corresponding tothe length of the cavity formed by the container members. Alternatively,it may cover only a small part of the length and be capable of beingmoved vertically during processing.

The coil 22 should be as close as possible to the container foreffective coupling, a practical spacing being less than Ms of an inch,indicated by x in FIGS. 3 and 4.

An inlet pipe 23 is provided in the base plate 15 and an outlet pipe 24in the top plate 18 such that, if required, a continuous gas flow may beprovided through the tube 14.

A means for holding the material to be melted and for transporting saidmaterial to the cavity 16 is provided and is illustrated as a hopper 25in FIG. 1.

As an example of the use of the above-described apparatus, the castingof a high-purity nickel alloy will be described. High-purity nickelalloy is fed via the hopper 25 into the cavity 16 formed by thecontainer members 10. Air within the region defined by the tube 14 andupper plate 18 and base plate 15 is evacuated and the region flushedwith argon. A steady flow of argon is maintained throughout theoperation of melting and casting, by means of the valves 26 controllingthe inlet and outlet pipes 23 and 24, respectively. Water is circulatedwithin the container members 10 and through the helical copper tubing22. The latter is coupled to a 15 kw., 400 kc. induction generator. Asthe power is increased, the nickel alloy is heated by eddy currentsinduced therein by radio-frequency currents flowing in the walls of thecopper tubing. The relationships among the primary radio-frequencycurrent 28, the secondary radio-frequency current 27, and the tertiaryradio-frequency current 29 are shown in FIG. 5. The molten nickel isheld within the cavity 16 by forces of surface tension and by theinfluence of the radio-frequency field. The cavity may be filled up to adesired level throughout the operation, or the hopper may be arrangedfor continuous feeding when the cavity is bottomless for continuouscasting if desired; if this be the case, means such as a valve or afurther evacuated chamber may be provided at the hopper so that gas flowWithin the tube 14 is uninterrupted. On completion of melting, theradiofrequency power is switched oif and the molten alloy allowed tocool and solidify in the shape defined by the cavity 16.

When the material to be melted has a resistivity too high forradio-frequency heating to be eflective, it may be necessary to preheatthe material to reduce the resistivity to a level suitable for meltingby radio-frequency heating.

As hereinbefore stated, the difiiculties in melting reactive materialsfor casting in most conventional refractory crucibles are mainly thoseof contamination by impurities.

4 For example, in certain cases an average of over 75% of castingsproduced from nickel-based alloys by melting in zirconia-type refractorycrucibles at temperatures in the order of 1600 C. have impuritycontamination, or gross solid inclusions from the disintegration of thezirconia crucible material.

The present invention has been found particularly suitable for themelting and subsequent casting of reactive materials. By utilizing anessentially cold, non-wetting container, of high electrical and thermalconductivity, the container itself being an inductance, contamination isavoided. The metals, copper, silver, gold and aluminum are particularlysuitable for the material of the container, because each of these metalshas high electrical and thermal conductivity. Silver is preferredbecause its electrical and thermal conductivity is the highest andbecause it can be most readily polished to reflect heat optically ontothe charge of material being melted. Alternatively, other metals havinga high conductivity may be used in place of silver and may besilver-plated.

Although the invention has been described in terms of a container formedfrom two members, it will be evident that, if necessary, a greaternumber of members may be used. The optimum number of members for anyparticular application will be determined partly by the impedancerequirements of the radio-frequency power source and partly by theprovision of an adequate circulation of the cooling fluid, and willusually lie between two and eight.

The invention may be applied to melting of materials capable ofresponding to radio frequency induction, Whether preheating is necessaryor not. For example, the invention may be applied to crystal pulling andsinglecrystal growth. When this type of container is placed in asuitable radio-frequency field supplied by an induction heater, thecontainer itself forms part of the work-coil inductance and thereforesupplies energy for melting the charge. Moreover, these containers maybe made in many forms depending on the particular application.

While the principles of the invention have been described with referenceto a specific embodiment and particular modifications thereof, it is tobe clearly understood that this description is by way of example onlyand should not be construed as a limitation on the scope of theinvention, which is defined by the appended claims.

What we claim is:

1. A method of melting material capable of being heated byradio-frequency induction comprising the steps of:

(a) placing the material in a container formed from a plurality ofmembers, each of said members being hollow and made of a metal having athermal conductivity of not less than 0.49 gram calories per cm. per cm.per deg. C. per sec. and a specific electrical resistivity of not morethan 2.665 microhms per cm. at 0 C., said members being electricallyinsulated from one another and so positioned in close proximity withrespect to each other that a cavity is formed within said container;

(b) circulating a cooling fluid through said walls;

(0) melting the said material by means of eddy currents induced thereinby means of radio-frequency electric currents flowing in the walls ofthe said con tainer; and

(d) on completion of the melting step, discontinuing said currents andallowing the said material to cool;

2. A method according to claim 1 wherein said container is surrounded bya medium having less than atmospheric pressure.

3. A method according to claim 1 wherein said container is surrounded bya medium of inert gas.

4. A method according to claim 1 wherein said material is continuouslysupplied to said container.

5. A method according to claim 1 wherein said material is chosen fromthe group comprising nickel, titanium,

zirconium, chromium, molybdenum, tungsten and uranium, and alloys andheat-stable compounds thereof.

6. A method according to claim 1 wherein said material is asemiconductive material.

7. A method according to claim 6 wherein said material is extracted fromsaid cavity while molten to produce a monocrystal.

8. A method according to claim 1 wherein said container is formed frommetals selected from the group comprising copper, silver, gold andaluminum.

9. An apparatus for melting material capable of being heated byradio-frequency induction including a container, formed from a pluralityof members, each of said members being hollow, said members each beingelectrically conductive and insulated from one another and so positionedin close proximity with respect to each other that a cavity is formedtherebetween, said container being electrically insulated from butinductively coupled to the coil of a radio-frequency current supply suchthat said container members act as secondary windings of a transformerof which said coil is the primary winding.

10. An apparatus according to claim 9, and further including means forsupplying said material to said cavity.

References Cited UNITED STATES PATENTS 4/1933 Northrup 13-26 8/1933Chesnut l3--26

